1. Field of the Invention
The present invention relates to a process for positioning spacer beads in flat panel displays such as microtip fluorescent screens, and a system associated with the process.
The present invention generally relates to the manufacturing of display screens comprising two adjacent plates made of glass, quartz or silicon and forming a flat airtight chamber that is subjected to a vacuum, and more particularly relates to the manufacturing of microtip fluorescent screens.
2. Discussion of the Related Art
Flat display screens require the use of spacers for maintaining a predetermined distance between the two plates supporting the electrodes constituting the device. These spacers raise a problem as regards their size (spacers must not De visible) and their implementation.
The microtip fluorescent screens are matrix screens formed by two sealed glass plates constituting a vacuum chamber whose thickness is controlled by spacers. The spacers must overcome the external pressure resulting from the internal vacuum and must provide an even thickness of the airtight chamber between the two plates. A conventional technology used in flat screens (including liquid crystals or plasma panels) provides depositing beads distributed over the whole surface of the screen, the beads having an average diameter equal to the distance between the two plates. In the case of liquid crystal screens, since the beads have a very small diameter (a few .mu.m), they are arbitrarily distributed on this surface. In the case of microtip fluorescent screens (like plasma panels), since the beads have a diameter of the same order of magnitude as the distance between the pixels, they must be accurately positioned at precise places where they have a minimum effect on the operation of the screen, for example, at the intersection of the lines arid columns. Moreover the number of beads must be sufficient so that each bead has a good resistance to external pressure.
In the prior art processes, the distance to be kept between the two plates is generally provided by beads that are distributed over one of the plates and glued thereon before the second plate is mounted. The distribution of the beads is controlled with a frame or an analogous device. Such a technique has many drawbacks. In particular, the positioning of the beads is not accurate which may cause some active portions of the electrodes disposed over the plates to be crushed.
For industrial manufacturing purposes, a process and a system, fast enough to comply with the assembly line requirements, are needed for positioning and depositing the beads.